Extraction-extrusion apparatus



R T Y O E V H O m m e m k QQ W W fl fin w m w vwmw N C. E N 7%? m Dec.18, 1956 E. c. BERNHARDT EXTRACTION-EXTRUSION APPARATUS Filed April 27,1954 United States Patent EXTRACTION-EXTRUSION APPARATUS Ernest C.Bernhardt, Montclair, N. J., assignor to E. I. du Pont de Nemours andCompany, Wilmington, DeL, a corporation of Delaware Application April27, 1954, Serial No. 425,985

3 Claims. (Cl. 18-12) This invention relates to a novelextraction-extrusion process and device, and in particular thisinvention relates to a method for removing solvent or other volatileimpurity from a synthetic plastic containing the same, and thusproducing an extruded melt of that plastic tree. of volatile impurity.More especially the method of this invention is well adapted for removalof small quantities of volatile liquid impurities, in the absence ofliquid con-v taminants other than these absorbed liquid impurities.

Many of the present day synthetic plastic materials are prepared in theform of solids having gaseous or liquid components absorbed (i. e.dissolved etc.).or occluded therein. Some plastic materials aresynthesized in the form of a slurry in which case the liquid mediummust-be removed in some manner if the plastic is to be prepared into asolid form such as granules or a sheet material or' the like. Some ofthe known synthetic plastics may be separated from at least a part ofsuch liquid by means of filtration, decanting, drying, and other meansknown to those skilled in the art. However, there are materials such asthe acrylic (including alkacrylic) polymers, synthetic linearpolycarbonamides of the nylon type, cellulose acetate, polystyrene,etc., which are prepared and in a form such that they contain liquidcomponents which are not readily removable by such means. For example,some of these polymeric products are hygroscopic, and require specialtreatment for removal of absorbed water before they can be extruded. Ifthis pre-drying step is not carried out, the moisture in the resincauses bubbles in the melt, as it leaves the extruder die. Otherpolymers, as

usually prepared, contain traces of monomers or miscel-.

laneous volatile impurities, which affect the quality of extrudates andwhich are sometimes dificult to remove. Accordingly, it would be highlydesirable if such compositions could be processed in a single operationto produce a melted extrudate which might be in the form of a sheet orother desired shape.

It is an object of this invention to provide an extractionextrusiondevice into which may be fed a mixture of polymeric material and avolatile component, and out of which there is obtained a meltedextrudate free of the volatile material which was present in the feedmaterial.

The accompanying illustrative drawing shows certain Y features of theextrusion-extraction device of this invention. In this apparatus, anextrusion screw (having special characteristics described below)cooperates with an extrusion barrel and an extruder feeding means in themanner shown in Figure 1.

Figure 1 is a cross section view of a housing containing an extrusionscrew hereinbelow described in detail. Figure 2 is a cross section viewof the portion of the screw showing the communication between theplastic material, i. e. the source of vapor, and the hollow core. Figure3 and Figure 4 show, respectively, the bubble-free extrudateformed bythe use of the extrusion screw of Figure 1 and Patented Dec. 18, 1956the extrudate containing bubbles obtained by the use of an ordinaryextrusion screw not equipped with the vaporwithdrawing core.

In Figure 1 the entire device is divided into four sections indicated bythe letters A, B, C, and D. The section designated as A is known as thefeed section. Section B is known as the metering or throttle section.Section C is known as the extraction section. Section D is known as theextrusion section. The apparatus consists essentially of a barrel 1,into which there is fitted a screw device 2, which is rotatably drivenin the indicated direction by a suitable power source connected at 3.The resin containing the volatile impurity is continuously fed intosection A through feed hopper 4. The resin is advanced by the rotationof screw 2 through sections A, B, C, and D in that order. By the timethe resin has reached section B, it is a viscous melted materialcontaining a small amount of the volatile material introduced intohopper 4 with the feed as absorbed impurity. In section B the rootdiameter of the screw has been increased to such an extent that thechannel through which the plastic travels is considerably reduced insize, whereby the flow of the plastic melt is throttled. It ispreferable that the root diameter of the screw be increased graduallyfrom some point in the center of section A to the entrance of section Balthough such an increase in the root diameter may take place if desiredover something less than a pitch length of the screw. At the entrance tosection C the root diameter of the screw is reduced, which makes thedrag flow capacity (defined at page 974, Industrial and EngineeringChemistry, May 1953) in section C greater than in section B. Generally,the drag flow capacity in section C is at least twice as great as thatof section B. Accordingly, the conveying capacity of section C isgreater than the capacity of section B. In section C there is a space 5in the forward part of the channel which the plastic material does notfully occupy. The space 5 is normally evacuated so that the melt leavingsection B will be subjected to a flash evaporation upon entering sectionC and, by reason of this evaporation, much of the remaining volatileliquid in the plastic material will be removed. If it is convenient ordesirable, the barrel 1 which surrounds the screw may be heated by steamcoils, electric coils, or the like. Section D is merely an ordinaryextrusion device wherein a preferably smooth barrel surrounds the screw2 and plastic material is forced through the section and out aconvenient die such as indicated at 14. The drag flow capacity insection D is greater than that in the metering zone, or at least equalthereto.

Section B is usually no less than about two pitch lengths of screw 2. Inthis section the root diameter of screw 2 is at its maximum value havingbeen increased to the diameter shown at 6. Such a restriction asindicated at section B may serve more than one purpose since thisrestriction throttles the flow, and may act as a metering device gaugingthe flow of plastic material through the entire extrusion device as wellas increase the pressure and temperature of the plastic material justprior to its introduction into section C shown in Figure 1. In sectionC, the material is subjected to a vacuum to remove virtually all of thevolatiles, or to lower the amount thereof to the desired concentrationin the final product. By increasing the pressure and temperature of theplastic material in section B and by sharply reducing the root diameterof screw 2 (shown in somewhat exaggerated fashion at 7) in extraction(vacuum) zone of section C the release of the volatile impurity isgreatly intensified by the sudden reduction in pressure. In effect,there is obtained a flash evaporation effect which speeds the removal ofthe remaining volatile liquid in section C. If it is desirable, barrel 1may be supplied with a means for heating such that the plastic materialcarried throughsections B and C may be heated to accomplish betterevaporation.

An essential and critical feature of the present invention is the means,provided for removal of vaporwfrom section C. It is surprising that byproviding an opening t3 communicating with the bore 9, in the extrusionscrew,

it becomes possible to apply a vacuum to the space.5 without danger ofplugging the exhaustlines. 'The opening 8 is located in the forward partof section C, which is free of resin in the vicinity-of-thesaid opening.The reason for this is related to the tendency of the device" toacculate resin at the rearward part of space-'5; leaving free space inthe forward portion. 7 i i The functioning of the means for-removingvapor from the .extractionzone C is-shown in more detail in Figure 2,which is a cross section view of a fragment of the screw 2, in theforwardgporn'on of 'sectionC, showinghow themoltenplastic is distributedtherein during operation of When the plastic'-first-- enters sectidnQitthe device. foams and spreads across the channel between threads asshown. at 10. As the foam-subsides untoamed 'resin collects in front ofthe screw. threadywith relatively little foamed resin spreading alongthechannel; in advancethereof, as shown at 11. Finally, before leavingsection C the resin is located almost entirely-immediately in'front ofthe thread,shown at 12, as aresult of'which-the port' 8 is kept: open.Asthe resin advances into section Dit is usually compressed so-as-tooccupy a'major part ofthe channel as shown at 13. I i v e Section D, asshown in Figure l; is-a pressure-zone (i. e. extrusion zone) which has adragflow-capaci'ty at least as great-as that of the metering zone, andgenerally less than that of the extraction zone. Thepressure zone;

Dcommunicatesdirectly with the extrusion die 14;

Figure 3 shows a clearplastic rod as extruded by use of. theabove-described screw. Figure 4 shows=a rod con-- taining. numerousbubbles, produced'by extrusion without the vapor-exhaustingmeans justdescribed.

In commercial practice methyl methacrylate polymer which has been freedof ingredients which cause bubbles on extrusion (this can be done byprolonged working on largewarm milling rollsl-is more expensive thanmethyl methacrylate polymer which has not been thus treated. The presentinvention permits the extrusion of thezlatter gradeofcommercialproductwithout bubble-formation, and without any significant increase inextrusion cost. In the commercial extrusion of nylon it is essential'touse driedproductwhich has been shipped in closed eans to assurecontinued dryness, for otherwise thelnylonpicks up water andbehaves inthe same manner as unmilled The present invention methyl methacrylatepolymer. facilitates the shipment and storage of nylonfor'extrusionpurposes by eliminating the need for'keeping the polymerintensively dried.

The following example serves to illustrate this invention.

. Example 1.jA' granular methyl methacrylate polymer which areidentical, except that one has a right-handthread device. Variouscombinations of variable and constant scribed. inuterms of a singlescrew and barrel, it is not intended that such limitation be imposedon'this ap paratusflor process; The features of this invention are;eq'uallyapplicable' to multiple'screw extruders. -Oneexample of such isa twin screw extruder, wherein two screws while the other has aleft-hand thread, are employed in a side-by-side relationship and arerotatedttoward each other in the nature of meshed gears or rollingmills. These multiple screws may be intermeshing or non-meshing withadjacent, screws.

Moreover, the vapor-withdrawal means providing communicationbetween thehollow core 9 and thespace 5 may bemorethanone port instead of only one,as

shown'in the drawing. The vapor-exhausting-meansmaybe a vacuum "pump orother similar vacuum -pro'ducings device; i."e.-,' device forwithdrawing gas or. reducingFthel pressure thereof;

The invention is notlimited to the employment at? pitch servestheintended purpose satisfactorily. In many embodiments of'thisinvention, however, it maybe ad l vantageousf to employ a screw with avariable pitch in,

one or more of the described sections of the extrusionpitch' th readswill 'be'apparent to those skilled in the art; forfexample; a screw witha variable pitch thread onascrew with; a variable pitch in thefeedsection andcompoundingisection and a constant pitch in thevacuum"extractionfs'elction' and extrusion section. These, and

other combinations, are intended to beincluded in the hei'eindescribedinvention.

fThe.extraction extrusion. device of this invention is useful intheprocessing of manygof a variety of plastic.

' materials where it is desired to blend plasticizers, pig

. that the exit of this device may' be fitted with any ofa]. variety ofmolds, dies, calenderingrolls' and other known deviceswhichare usefulintransforming 'a molten plasticj composition containing traces ofmoisture, monomer and mercaptans was found to give, on extrusion withanormal' screw, a product which contained bubbles. Even when this sampleof granular polymerwas dried for prolo'nged' periods of time at 70 C. itcould-not be extruded-at The materials which may be processedbytheextrusion device of this inventionmay be any set aivariety. ofsynthetic plastic materials which are produced with ocmerits", fillers,and other-materials with a polymer in a; dispersion formand to producean extrudatereadilyfor' molding into, a desired shape. It willbeappreciatedf into sheets, rods, tubes, and articles of anyshapewhats'cn, ever. a

volatile compon'ent.from a mixtureof a plastic material.

containing said volatile component, comprisinga-cylin drical. barrel, ahelical screw conveying. device rotatably mounted in said barrel and ameans for driving .said screw. conveying device, said screw having azoneaof,

greater drag-flow-capacity following a throttling one,

oi-lesseradragflow capac1ty, said screw havinga hollow.corecommunicatingwith the said zone of greater dragew eaa i y and w 3 9r ap in acuum:

thereto, said;communication-being positioned in the f r ward halfoff;the-zoneof greater drag-flow; capacity, and; in the gfor ward half ;of achannel between threads in the, said zone- 2,; apparatus according-toclaim 1 1 wherein there I is. a' channel between the said screwandthesaid-barrel:

which, comprises four-,zones,. namely, (1) a feed zone,cornmunicatingwith a means for introducing a: mixture: of plasticcontaining'volatile impurity, (2) a'zonetwhich throttles thefiow from;the feed zone and governs. the.

output rateof the extruder," (31) :anextraction zqnewhich has adrag-flow capacity greater than that of the meter- 117,.A11extrusion-extraction apparatus for removing,

zone.

3. An apparatus according to claim 2 wherein the said screw is ofconstant pitch.

UNITED STATES PATENTS Price Oct. 12, 1915 Gerstenberg Apr. 28, 1936Weinberg Oct. 31, 1950 Fuller Oct. 28, 1952 Corbett June 15, 1954

